Window mounted antennas have gained more and more popularity in mobile radio links, especially in cellular telephone communications because of their obvious advantages to the consumer. These advantages include the ease of installation and the fact that it is not necessary to drill a hole in the vehicle. Many efforts in designing effective window mounted antenna systems have been disclosed in the patent literature. The majority of these are capacitively coupled systems. With the introduction of PCNIPCS (Personal Communication Network/Personal Communication Services), capacitive coupling becomes troublesome due to the doubling of the frequency and bandwidth requirements.
U.S. Pat. No. 4,089,817 to Kirkendall illustrates one capacitively coupled antenna system for use with half wavelength antennas. U.S. Pat. No. 4,839,660 to Hadzoglou discloses another capacitive coupling system--this one for use with a bottom radiation element of between 1/4-wavelength and 1/2-wavelength. (Hadzoglou's bottom radiation element cannot be a full dipole because of the high transition impedance sensitivity at a 1/2-wavelength.) U.S. Pat. Nos. 4,992,800 to Parfitt, 4,857,939 to Shimazaki, and 4,785,305 to Shyu, follow similar principles, all involving LC matching networks and capacitive coupling through the vehicle glass.
Capacitive coupling systems (e.g conducting patches mounted on opposing sides of window/windshield glass to form a capacitor coupling RF energy therethrough) suffer from a number of disadvantages, summarized below:
1) To present a substantially capacitive reactance, the coupling patches cannot be large in comparison with the operating wavelength. High impedance coupling (several hundred ohms) results, leading to losses through the leakage of electrical field at high frequencies. PA1 2) In the higher UHF bands, such as the 1.5-2.4 GHz frequencies used for PCN/PCS/ISM services, even a "small" coupling patch does not behave as a lumped capacitor element. Considering the thickness of vehicle glass and stray capacitance, the coupling circuit can bypass the signal and make it more difficult to match the high impedance of the antenna to a 50 ohm system. PA1 3) The high impedance coupling afforded by capacitive coupling creates a moisture sensitive structure. U.S. Pat. No. 4,764,773 to Larsen describes a better coupling structure to improve performance in the presence of moisture, but it is still subject to patch size limitations. PA1 cable.fwdarw.microstrip.fwdarw.slot.fwdarw.glass.fwdarw. PA1 slot.fwdarw.microstrip.fwdarw.i.m.n..fwdarw.antenna
In addition to problems with capacitive coupling systems, the conventional collinear array antenna presents problems of its own. For example, such antennas do not have uniform current distributions; the lower section of the whip exhibits the strongest radiation. In most vehicle mounting situations, the lower section of the whip is blocked by the roof of the vehicle, causing severe pattern distortion and deep nulls. This situation becomes worse in the 1.7-2.4 GHz PCS/PCN/ISM bands simply because the length of the radiator is less than half that at the 800 MHz cellular band due to the more than doubling of the frequency. To reduce this problem, elevated feed systems are sometimes employed. But antennas with elevated feeds are not easily matched for broadband operation (e.g. up to 11% for DCS-1800). Moreover, such elevated feed systems often present a low impedance (e.g. 50 ohms) at the through-glass coupling point, limiting the through-glass coupling techniques that can be used. If traditional capacitive coupling is employed, a matching network must, somewhere, be employed to transform impedances. Such matching networks tend to have prohibitive losses at the high UHF frequencies of the PCN/PCS/ISM services (typically 4-6 dB).
U.S. Pat. No. Reissue 33,743 to Blaese describes a different capacitively coupling system for coupling a coaxial cable through the glass. But at the PCN/PCS/ISM frequencies, the quarter-wave antenna employed by Blaese would be only 1.7 inches long--completely below the roof line of a vehicle, causing severe pattern distortion and deep nulls. U.S. Pat. No. 4,939,484 to Harada discloses a coupler comprising helix cavities for through-glass coupling. While suitable for use in the 800 MHz cellular band, this arrangement has a number of drawbacks when scaled to the 1.8 GHz PCS band. For example, the coupling aperture becomes undesirably small. Moreover, the helix Q is relatively small due to the size of the helix. Still further, the coupling coefficient is too small to provide adequate coupling over the wide (11%) PCS band. Manufacturing and tuning are complicated by the high frequency and the coupler's complex 3D structure.
Most of the above-discussed drawbacks are present with other through-glass couplers described in the prior art (notwithstanding the prior art's laudatory assertions of their general applicability at frequencies above the 800 MHz cellular band).
Accordingly, there is a need for an improved method of through-glass (or through other dielectric) coupling for use at gigahertz frequencies.
One attempt to meet this need is disclosed in my U.S. Pat. No. 5,471,222. The disclosed system employs microwave cavities containing high Q ceramic resonators, with RF signals fed through the glass by a pair of TE.sub.01.delta. mode dielectric resonators.
The disclosed approach is highly efficient, with an insertion loss of 0.5dB (through 5 mm automobile glass at 1.8 GHz) attainable with careful tuning. However, this design is expensive to manufacture and is sensitive to detuning in the field.
Another attempt to meet this need is disclosed in my U.S. Pat. No. 5,451,966. In that system, a rectangular slot coupling scheme replaces the expensive ceramic couplers of my '222 patent. (The concept of slot coupling on a microstrip antenna (MSA) is understood to have originated with Pozar. See, e.g., his publication "Improved Coupling for Aperture Coupled Microstrip Antennas," Elec. Lett., Vol. 27, pp. 1129-1131, June, 1991.) Slot coupling is used to overcome the narrow band nature of MSA. A "doggie bone" type of slot, suggested by Pozar, significantly increases the magnetic polarisability on the slot, allowing a short slot to provide the necessary coupling while at the same time keeping the backward emissions low. Pozar and other researchers' work has generally been limited to numerical solutions of slot-fed microstrip antennas and multilayer arrays on a ground plane. But the bandwidth advantages of this type of MSA can be used to enhance the concept of the planar slot-cavity coupler. Furthermore, recent progress in low cost, high performance microwave printed circuit board material has brought about the opportunity to make this type of antenna system affordable for commercial applications. Based on this MSA process, a "doggie bone" type slot coupled antenna system was developed with the coupling element etched on low loss Teflon.TM. PCB and it has proven to be quite successful in the field.
Unexpectedly, I have discovered that a simpler and less costly coupling technique is capable of achieving the same superior performance of the previous arrangement, while at the same time providing various advantages over the rectangular slot approach.
One issue in the existing slot-coupled approach is cascade coupling, which can be diagrammed as:
Another issue is the so-called "MSA effect." The E field excited by a rectangular slot is always distributed perpendicularly to the slot, making the opposite coupler an antenna patch. The inner and outer PCB, however, must be limited in size to satisfy the resonant frequency. This introduces a substantial loss inherent in all slot-fed variations of the MSA.
Moreover, radiation always occurs at the edges of the resonant direction of the patch (i.e. perpendicular to the slot) by means of an equivalent magnetic current represented as M=EXn. The presence of a larger ground plane supports the tangential portion of the E field. When a rectangular slot is used as a glass coupler, the edge E field still exists, leading to a radiation loss. In the previous art, the lengths of the two ground planes on the PC board are selected and aligned in the resonant direction to form a glass mount antenna. The MSA effect is obviously observed.
Finally, to achieve a high coupling coefficient, long slot lengths arguably should be used. But this presents the problem of increasing backwards radiation.
In accordance with the preferred embodiment of the present invention, through-glass coupling is achieved with an annular ring type aperture coupling arrangement. One advantage of this approach over rectangular slot coupling is that it raises the coupling coefficient, which is important for coupling through a relatively thick dielectric wall. Another advantage is that the radial distribution of the E field from an annular ring aperture tends to increase the aperture coupling and reduce edge coupling.
The annular ring aperture coupler of the present invention also aids the issue of backwards radiation from the slot itself. FIG. 2 presents an estimated radiation resistance of an annular ring slot according to the preferred embodiment. For a rectangular slot, as mentioned by Pozar and other researchers, the backwards radiation of a slot-fed MSA can effectively be cut by shortening the slot length and end-loading the slot to retain a sufficient coupling coefficient. This technique can also be applied to glass couplers; An annular ring is the complementary element of a small loop antenna and, like the loop antenna, presents a low radiation efficiency, but this effect is here turned to advantage by reducing backwards radiation. A larger E field aperture can be achieved, with less MSA effect. An impedance matching network is avoided by connecting the CPW line directly to the center resonant element instead of using a transition coupling scheme, as described in the prior art. With this improvement, the i.m.n. stays in the same layer as the resonant element, facilitating fabrication (e.g. a single layer PCB or simple stamped metal parts).
By the foregoing arrangement, the loss mechanisms of the prior art are largely eliminated, leaving just the dielectric loss of the vehicle glass. Results like that of the ceramic coupler arrangement are thus achieved, without its cost, manufacturing, and detuning drawbacks.
One object of the preferred embodiment is thus the provision of a cost effective glass mount antenna system operating at frequencies higher than the existing cellular band.
Another object is the provision of a through-glass coupler that is simpler than the prior art, facilitating mass production and lowering manufacturing costs.
Another object is the provision of a through-glass coupler operating at relatively low impedance while enabling a high feeding point and providing broadband operation.
Another object is the provision of a through-glass coupler that minimizes loss factors present in the prior art.
Another object is the provision of a through-glass coupler that reduces backward radiation while maintaining a high coupling coefficient.
Another object is the provision of a through-glass coupler that reduces edge-coupling effects of the prior art.
The foregoing and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.